Allow inserting inputs/outputs at specified positions

While the positions of inputs and outputs don't usually matter, they do
matter in covenant constructions. This allows easier construction rather
than creating a new vec or swapping things around

src/pset/mod.rs

@@ -100,6 +100,14 @@ impl PartiallySignedTransaction {
         self.inputs.push(inp);
     }
 
+    /// Add an input to pset at position i. This also updates the
+    /// pset global input count
+    /// Panics if index is more than length.
+    pub fn insert_input(&mut self, inp: Input, pos: usize) {
+        self.global.tx_data.input_count += 1;
+        self.inputs.insert(pos, inp);
+    }
+
     /// Read accessor to inputs
     pub fn inputs(&self) -> &[Input] {
         &self.inputs
@@ -127,6 +135,14 @@ impl PartiallySignedTransaction {
         self.outputs.push(out);
     }
 
+    /// Add an output to pset at position i. This also updates the
+    /// pset global output count
+    /// Panics if index is more than length.
+    pub fn insert_output(&mut self, out: Output, pos: usize) {
+        self.global.tx_data.output_count += 1;
+        self.outputs.insert(pos, out);
+    }
+
     /// read accessor to outputs
     pub fn outputs(&self) -> &[Output] {
         &self.outputs